Many laser systems employ process beams that have radiation wavelengths that are outside of the visible range of radiation wavelengths that extends from approximately 450 to 750 nanometers. The process beams for such laser systems are used, for instance, in photolithography applications for the fabrication of microminiature structures, such as integrated circuits, wherein the process beam is ultraviolet, and in medical applications, wherein the process beam is infrared.
The targeting and positioning of such invisible process beams present special problems with respect to monitoring and controlling process beam spot size and position on a work area, particularly if the process beam must be moved across the work area during a processing operation. To allow adjustment of the process beam spot size and realignment of the process beam position during a processing operation, it is common practice to optically combine the beam of a visible laser, or the beam of an invisible laser that is viewable with a device such as a CCD video camera, with the process beam in such a manner that both beams have a similar spot size and position on the work area. They are also combined in such a way that any adjustment to the visible beam spot size or position causes a substantially corresponding change in spot size or position to the process beam.
Laser systems that incorporate such visible alignment beam provisions are generally bulky, complex, and unstable because of the number of optical elements that are used to combine the beams. For instance, the laser for the process beam generally has a lens assembly for converging and shaping its beam and the laser for the alignment beam has an independent lens assembly for converging and shaping its beam. The independent lens assemblies are generally necessary because of the different radiation wavelengths of the two beams as well as a probable difference in different divergences for each of the two beams from their laser sources to the work area.
Some means for combining the process and alignment beams is also necessary, such as with one or more totally reflective mirrors and a dichroic mirror, or a similar arrangement. An achromatic lens assembly may also be used to change the convergence and shape of the combined beams before they reach the work area. The change in position or disturbance of the dimensions of any of these elements, such as caused by thermal expansion or contraction, can cause misalignment of the entire laser system. Such misalignment can cause damage to the work area as well as cause excessive time loss during realignment of the laser system.
It is therefore desirable to use a laser system with an invisible process beam and a visible alignment beam that has fewer optical elements and is less prone to misalignment.